Document Rp1bvDvMKv2g8rzp5XdjELkq7

American Society of Heating and Ventilating Engineers Guide, 1929 above the machine and where a heavy vapor or dust-laden air at ordinary temperature is to be removed, horizontal or floor connections are re quired. If it is attempted to remove heavy dust such as lead oxides by an overhead hood the conditions may be worse than if no exhaust were used at all, owing to the rising air current carrying the dust up through the breathing zones. The principle to keep in mind in all cases is to take advantage of the natural tendency of the material to move upward or downward. In another class of operation the main object is to prevent the escape of dust into the surrounding atmosphere, the removal of some dust from the machine or enclosure being merely incidental. The dust creating apparatus is enclosed within a housing which is made as tight as prac ticable, and sufficient suction is applied to the enclosure to maintain, an inward air leakage, thus preventing escape of the. dust. While the ex haust system is only required to handle the air which leaks in through the crevices and openings in the enclosure,' yet in many 'installations leakages are very high and great care is required to obtain satisfactory results with a system of this kind. The inward leakage principle is utilized for controlling dust in the operating of tumblingbarrels, grinding, screen ing, elevating and similar processes. Certain dust and fume producing operations are best carried on by isolating the process in a separate compartment or room and then apply ing general ventilation to this space. The compartment or room in which the work is performed should be as small as is consistent with convenience in handling the work. The ventilating system should be designed so that a strong current of Clean air is drawn across the operator, and away-from him toward the work, where the dust is picked up and carried from the room. IMPORTANT REQUIREMENTS OF AN EFFICIENT EXHAUST AND COLLECTING SYSTEM It is impracticable to enumerate all of the requirements for an efficient exhaust and collecting system, however, among the more important are the following: 1. Fans, collectors, hoods, and ducts should be of adequate size. 2. Air volume and velocities should be adequate for the work to be accomplished. 3. The exhaust hoods should not interfere with the operation of the machine or access to its working parts. 4. The system should not increase the fire'hazard. 5. The system should not increase the dust explosion hazard. 6. Where power is expensive, it should do the required work with a minimum power consumption. 7. In cold climates, it should not remove any more air than necessary, from the building. 8. Where-power is comparatively cheap, first cost should be low, even if the power required to operate is slightly higher. The first step in designing systems employing hoods to trap the material is to determine the number and size of the connections for each individual machine. At this point the designer's past experience is of great value,' as, while it is possible to set certain general standards, yet in actual practice the sizes are considerably affected by the local conditions which the 462 l Chapter XXXII--Dust, Exhaust and Collecting Systems layout man finds in the field, and he bases the pipe sizes and hoods on his judgment, being guided by his experience and the general practice; The size of hoods and connections are determined by the size and type of machines or apparatus to be handled by the exhaust system, by the kind of material worked, by the duty of the machines and other local conditions. It is impracticable to lay down any general, rules for de termining size connections for the various types of machines and Tables 1 to 3, giving sizes as used in some of the common industries are only intended to serve as a general guide. Under certain favorable conditions smaller connections may be supplied. Open bottom exhaust hoods of the canopy type, where it is impractical to enclose completely the point of origin of the dust or fumes, should extend over the machine or operation at least 12 in. in every direction if the hood is not elevated more than 2 ft. For each additional 2 ft. of elevation, the size of the hood should be increased 12 in. in all directions; It is.desirable to make the area of the connecting pipe not less than ts of the total hood area. In systems employing inward air leakages the area of connection must be proportional to total leakage area in the enclosing housing. Tumbling barrels have connections ranging from 4 to-8 in., bucket con veyors 6j to 12 in., and screening machines 6 to 10 in.. In general room exhaust large connections should be provided so that the air: may be handled at low velocity and with a minimum power con sumption. After having determined on the proportions of the exhaust system as regards hoods and connections it is then necessary to choose the air velocity or suction at the hood connections, suction at the hood con nections being a measure of the air velocity at that point. AIR VELOCITY The air velocity required is dependent upon the specific gravity of the material, the fineness of the particles, and their physical characteristics. Certain materials such as grease wools, silk waste, salt, and other hydro scopic substances are difficult to handle due to the tendency to deposit in the conveyor pipes. While the velocity in the system should be sufficiently high to insure the removal of the material it should be kept as low as practicable since any higher velocity requires the use of unnecessary power. With a fixed system or orifice the power increases as the cube of the increase in velocity, or'fdr the same 'quantity of air and material handled', inversely ,as the fifth power of the diameter of round ducts. J Velocities commonly employed are: 2500 to.3000 ft. per minute.for light dusts, cotton, shavings and sawdust from dry wood, and similar substances. Heavy dusts, wool, shavings and sawdust from wet wood, rags, waste paper and similar materials 3000 t6 4000 ft. per minute. Lead dust, hog waste, pulp chips, etc., 4000 to 6000 ft. per minute. In choosing the pipe sizes consideration must be given to the way and manner in which the machines will be operated, as in case a considerable number of machines, all discharging into one main, should be shut off 463